1. Field of the Invention
This invention relates to aircraft aircrew life support systems and is more particularly concerned with a system which includes an aircraft on-board oxygen generating system for delivering oxygenenriched breathable gas.
2. Description of the Prior Art
On-board oxygen generating systems (OBOGS) utilising molecular sieve adsorber beds have been demonstrated to be advantageous to the safety and operation of a wide range of military aircraft. As a result it is now common practice in aircrew life support systems to supply oxygen-enriched breathable gas to the aircrew from an OBOGS.
In some systems breathable gas delivered by the OBOGS is supplied by way of a breathing demand regulator to a face mask worn by an aircrew member so that effectively he is breathing gas direct from the OBOGS. In such systems breathable gas flow from the OBOGS must be such as to cope with peaks in breathing demand. This may require flows as high as 200 liters/minute so that the molecular sieve beds of the OBOGS must be of considerable volume making greater demands on the space required for housing the OBOGS within the aircraft.
Also, it is usual for the OBOGS to derive its supply air from a bleed of aircraft engine compressor air so that it is operational only when the engine is running. This gives rise to a requirement that provision be made to store breathable gas on the aircraft for use by an aircrew member when he is sitting in his aircraft on the ground before engine start-up or after engine shut-down, or in the event of failure of the supply of compressed air.
Systems have been proposed which incorporate a plenum storage tank charged with breathable gas delivered by the OBOGS at the operating pressure of the system. Whilst this is a solution, it can be shown that the minimum requirement is in the order of 600 liters of stored breathable gas at normal temperature and pressure which requires a large volume storage tank be housed within the aircraft.
It is also necessary for breathable gas to be supplied to the aircrew member during bail-out from the aircraft in the event of an emergency. This requirement is normally met by providing a pressurised breathable gas bottle, normally containing 100% oxygen, mounted on the aircrew seat which gives rise to an additional servicing task.
US-A-4428372 (issued to Linde and now assigned to the present applicant) discloses an aircraft aircrew breathing system in which a portion of oxygen-enriched breathable gas delivered from molecular sieve adsorber beds of an OBOGS is stored in an emergency storage tank for use in the event of an interruption of the normal supply of breathable gas from the adsorbers. Whilst the emergency storage tank provides a back-up supply of breathable gas for use by aircrew within the aircraft, the provision of breathable gas to the aircrew during bail-out from the aircraft is neither disclosed or discussed.
US-A-4651728 (Boeing) discloses an aircraft aircrew breathing system which includes an OBOGS for supplying oxygen-enriched breathable gas for breathing by aircrew. This system has a standby supply of breathable gas and an emergency supply of breathable gas stored in separate cylinders. A selector valve is provided whereby breathable gas may be withdrawn from the standby cylinder for breathing by aircrew when the aircraft is on the ground with the OBOGS non-operational or during flight if the OBOGS malfunctions. The emergency breathable gas cylinder is included primarily as a source of breathable gas for use by the aircrew during bail-out from the aircraft but may also be used in the event of failure of both the OBOGS and the standby cylinder.
In one system embodiment disclosed in US-A-4651728 the standby cylinder is charged with oxygen-enriched breathable gas delivered by the OBOGS. Gas pressure in the storage cylinder is sensed and when it falls below a designated value the OBOGS is switched to produce breathable gas of high oxygen concentration which is compressed by a compressor before being fed to the storage cylinder. When the storage cylinder has been recharged to a preselected pressure the OBOGS returns to its normal operation and the compressor is switched off.
There is no disclosure in US-A-4651728 as to the method by which the emergency breathable gas cylinder is charged. In the absence of any disclosure to the contrary it must be assumed that the emergency cylinder is charged externally of the aircraft with 100% oxygen gas under pressure in the manner of conventional emergency oxygen bottles used on aircraft at the present time.
A disadvantage of both the systems disclosed in US-A-4428372 and US-A-4651728 is that in normal operation oxygen-enriched breathable gas is supplied direct from the OBOGS to the aircrew with the standby or emergency gas sources being used only in the event that the OBOGS is non-operational or malfunctioning. This gives rise to the requirement that the molecular sieve beds of the OBOGS be sized to cope with peaks in aircrew breathing demand thereby increasing the space envelope required for housing the OBOGS in the aircraft.